1. Field of the Invention
This invention relates to antennas and more particularly an antenna that uses cross-polarization with either a ground plane or no ground plane to provide enhanced telecommunications or the like.
2. Description of the Related Art
All forms of radio or similar telecommunications require an antenna in order to transmit and receive radio waves and the like for communication. With increasing cellular communications and short-distance telecommunications, antennas are becoming more a part of the commonplace environment. Particularly with cellular telephones, the power supplies for the antenna associated with the cellular phone is powered by a battery and is consequently limited in power and duration of the power supply. Due to these power and other limitations, it is important to provide an antenna that maximizes the efficiency of the available power, to transmit a clear signal as far as possible.
Stationary and other antennas, such as those mounted on cars and the like, are generally within easy reach of passersby or pedestrians. Such easy access makes such antennas often subject to vandalism or other unwanted attention. By making such antennas as inconspicuous as possible, undesired attention can be avoided and the useful life of the antenna can be extended. In order to achieve low visibility, the antenna must achieve a compact size through packaging and possibly disguised or non-traditional antenna shapes.
In the art, it is known that destructive interference occurs when reflected signals destructively interfere with transmitted signals. This is known Raleigh fading and creates signal fading or dead spots that inhibit or diminish the desired communications for which cellular phones and the like are intended. In designing an antenna meant for daily or commonplace use in a cellular or similar environment, an advantageous antenna design avoiding Raleigh fading is not currently available and is something that would well serve the advancement of the telecommunications arts.
In order to decrease the apparent size of a monopole antenna, the antenna can be shortened by making the antenna in the shape of a spring, or coil, by winding it around a cylindrical core in the manner of a helix or otherwise. Such helical antennas are described in detail in Kraus, Antennas, Chapter 7, pp. 173-216 (McGraw Hill 1950) and in a number of U.S. patents. A practical example of a linearly polarized antenna may be found in the ARRL Antenna Handbook, "Short Continuously Loaded Vertical Antennas," pp. 6-18 to 6-19 (Gerald Hall ed., ARRL Press 1991).
Helical antennas may be made from wire or metal tape wrapped around a cylindrical core made of plastic or plastic-glass composite. In winding the antenna around the core, the length of the antenna and the pitch at which it is wound around the core are fashioned so that the resulting antenna is resonant at a desired frequency. A shortened antenna has the radiation resistance and consequent narrow band width of a straight length wire of the same length. However, with the coiling of the wire about the core, an inductance is introduced that approximately cancels the series radiation capacitance of the equivalent short wire antenna.
The narrow bandwidth of such inductively shortened antennas can be used to good effect at frequencies below 30 MHz, where they enjoy frequent use. However, at higher frequencies, wider bandwidths are required and the narrow bandwidth of such antennas prevent them from being used at such higher frequencies. In order to compensate for the narrow bandwidth of the inductively-shortened antenna, common practice includes tuning means so that the frequency may be tuned by either expanding or contracting the length of the helix, or by adding resistances in series with the low radiation resistance of the antenna. This is shown in the patent to Simmons, Broadband [Helical] Antenna (U.S. Pat. No. 5,300,940 issued Apr. 5, 1994). By accommodating and compensating for the narrow bandwidth, an improvement is made in the apparent bandwidth in the VSWR (voltage standing wave ratio) of the antenna but at the expense of radiation efficiency. Of course, radiation efficiency is especially important for battery-powered transmitters and for those transmitters that are a significant distance (near the periphery of the transmitting range) from a cellular or other receiver.
Where tuning is impractical and/or where high efficiency is required, some additional bandwidth may be gained by making the helix larger in diameter thereby increasing the width to length ratio. However, as mentioned in the Kraus reference above, as the diameter of the helix is increased and as the pitch and length of the turns are adjusted to maintain the resonance of the antenna, the polarization of the resulting antenna changes from dispersive linear radiation to endfire circular radiation. This change of direction of radiation from broadside to endfire is generally impractical for mobile and portable applications. Such high directivity and such an unfavored angle of radiation impose certain inconveniences and limitations upon small transmitters and their antennas. However, there are some uses for an endfiring helical antenna such as those which are described in the patent to Wheeler entitled Antenna Systems (U.S. Pat. No. 2,495,399 issued January 1950).
Field diversity, that is the diversity in the polarization of the vertical and horizontal field components, is known to address and to help resolve Raleigh fading. K. Fujimoto and J. R. James, Mobile Antenna Systems Handbook, pp. 78-85 (Artech House 1994), A. Santamaria and F. J. Lopez-Hernandez, Wireless LAN Systems, p. 180 (Artech House 1994). The advantages arising from cross-polarized radio signals is also addressed in "Experimental Results with Mobile Antennas Having Cross-Polarization Components in Urban and Rural Areas," Kuboyama et al., IEEE Transactions on Vehicular Technology, Vol. 39, No. 2, May 1990, pp. 150-160. Field diversity, or cross-polarization, results when the horizontal and vertical field components of the radiated signal are radiated in phase. This is in opposition to circular polarization, which occurs when the horizontal and vertical field components are plus or minus 90 degrees out of phase and to the situations where only horizontal or vertical field components are present exclusively.
In order to obtain field diversity from an antenna, particularly a helical antenna, the helical antenna must be dimensioned between its linear and circular polarization modes in order to achieve field diversity. One such helical antenna is illustrated in FIG. 1 of the patent to Halstead, Structure with an Integrated Amplifier Responsive to Signals of Varied Polarization (U.S. Pat. No. 3,523,351 issued August 1970). As an alternative to the helical structure of the antenna, meander lines can be used as set forth in the patent to Drewett, Helical Radio Antenna (U.S. Pat. No. 4,160,979 issued Jul. 10, 1979). Radomes are also known in the art per the patent to Frese, Helical UHF Transmitting and Receiving Antenna (U.S. Pat. No. 5,146,235 issued Sep. 8, 1992).
Despite the established art and current developments thereof, the use of field diversity in a small antenna for cellular or similar use is not known in the art. Additionally, such antennas would provide significant advantage as radio telecommunications could then also take place in conjunction with a variety of different objects such as vending machines, as well as individuals with their cellular phones and other electronic data and information machines. To achieve greater utility, such an antenna should function well with or without ground planes and should provide impedance matching and compensating circuitry to maximize the bandwidth of the antenna.